Abstract

Abstract
The standard molal thermodynamic properties of α-GaOOH at temperatures to 700 K were retrieved from (1) low-temperature α-GaOOH heat capacity measurements, (2) solubility measurements ofα-GaOOH and ß-Ga2O3, and (3) data available for the α-GaOOH - ß-Ga2O3 transformation in the presence of aqueous solution. The heat capacity of α-GaOOH as a function of temperature was found to be consistent with the formulation CP° = 54.38 + 0.06418 T - 11.3 105/T2 (J/mol.K) in the temperature range 298-700 K. The estimated standard molal entropy of α-GaOOH from low-temperature heat capacity measurements is 51.61 ±0.26 J/mol.K. The estimated standard molal Gibbs free energy of formation from the elements for α-GaOOH as retrieved from α-GaOOH and ß-Ga2O3 solubility experiments and available stability data was -627.9 ± 2.0 kJ/mol. The corresponding α-GaOOH standard molal enthalpy of formation from the elements was found to be -705.3 ± 2.5 kJ/mol. These data allowed the prediction of the thermodynamic properties of α-GaOOH to high temperatures and demonstrate that α-GaOOH and ß-Ga2O3 are the stable solid phases in the Ga-O-H system at saturated water vapour pressure below and above 573 K, respectively